Half-Life and Left 4 Dead developer Valve has been toying with new ways to use biofeedback—heart rate, facial expressions, eye-tracking, levels of physical arousal—for years. What does that mean for the future of people who like Valve video games? How about controlling games with your eyes?

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Valve's Mike Ambinder talked about the sundry ways the company is experimenting with incorporating user biofeedback at GDC today. Currently, that includes prototype versions of Left 4 Dead 2 with an "AI Director" that responds to your arousal levels, modifying how it distributes health items, zombie hordes and Special Infected based on "player trauma."

It also includes a build of top-down shooter Alien Swarm that alters the game's timer in response to stress and a version of Portal 2 that decoupled aiming a cross-hair and viewing the game world.

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Ambinder showed a demo of a Left 4 Dead 2 player connected to a biofeedback measurement device, a custom piece of hardware designed to detect skin conductance response. An in-game graph displayed levels of arousal from the player as he fought a series of Special Infected zombies, trying to refill a generator with gas cans. The player's stress levels steadily increased as he was attacked, until ultimately he peaked during a Tank battle. A post-gameplay graph showed spikes in player trauma levels, tied to game events: Smoker attack, Charger battle, game-ending Hunter pounce.

A Left 4 Dead 2 prototype used player biofeedback in another capacity, showing a teammates' arousal levels above their health bar to illustrate how other Survivors were responding to in-game trauma.

Ambinder said that data was entertaining to watch during competitive play in the upcoming DOTA 2. This, he said, was "the most enjoyable thing we've done so far" with biofeedback. Players who saw their opponents' sense of arousal spike would "go crazy" with delight.

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"It's great to watch people suffer because of your actions," he said.

Ambinder also showed gameplay of Portal 2 being controlled with eye-tracking. Player movement, looking around the game world, was still done by hand, but aiming with a crosshair was done by looking with the eyes—they move faster than the wrist, so would using them to aim be an improvement? Enjoyable?

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Eye-tracking hardware determined the X/Y location of a gaze, redrawing the targeting reticule at 60 Hz. It was jittery, but speedy, leading Valve to determine that eyes could be viable aiming controllers. Decoupling the aiming viewpoint was a plus, Ambinder noted.

Biofeedback controllers may be a long ways off, Ambinder said, cautioning developers that consumer grade eye-trackers are still "far away" due to cost. Clearly, though, Valve sees value in factoring in physical feedback into its games and, at the very least, watching them experiment is highly entertaining.